In gas tungsten arc welding, sometimes referred to as TIG welding, the output current can be provided by any number of constant current power sources. Most metals are capable of being welded by the GTAW or TIG welding process; however, they are generally welded by a DC negative arc using an inert gas atmosphere. When welding aluminum or magnesium using the TIG welding process, the current is generally AC with an electrode positive current section and an electrode negative current section. These two sections are balanced to a limited extent using known technology; however, more recently, variable polarity power sources have become available. Using inverter technology with a waveform generator, as pioneered by assignee of the present invention, the positive current section and the negative current section of the waveform for TIG welding can be adjusted in shape, amplitude and duration. AC TIG welding of the variable AC waveform type is performed with a pure tungsten electrode. The size of the electrode is determined by the application and the current range required for the welding process. If a given size electrode is welded at a positive current or positive heat energy above its desired range of operation, the tungsten electrode may become too hot and the electrode will “spit” metal from its molten tip into the welding pool. This is not acceptable. In most cases, the electrode spitting occurs in the positive half cycle or section of the waveform when the instantaneous positive current is too high for the particular electrode being used. In common AC TIG welding, duration of the different current sections are controlled for adjusting the welding process to the desired conditions. This duration adjustment is a direct linear relationship with greater negative penetration durations and greater positive clean durations as the set current is increased. Such control does not address the problem of electrode spitting by overheating the tip of the electrode.
TIG welding with an AC waveform using the inverter type technology is performed by using a square wave positive current section and a square wave negative current section. The average current for the TIG welding is generally obtained by multiplying the positive current by the positive duration and the negative current by the negative duration. These functions are then added and divided by the total duration of both current sections. When performing a TIG welding process, the average current is generally set by the operator or adjusted by a foot pedal. The set current is maintained by controlling the current during both the positive and negative portions of the AC waveform. However, it is not unusual to adjust or set the current based upon the current during the negative polarity waveform, since this portion of the waveform is used for heating and penetration while the positive portion is merely used for cleaning the workpiece. To provide desired TIG welding results, as mentioned earlier, the cleaning duration is adjusted in a direct relationship with respect to the penetration duration based upon the desired TIG welding current, whether the average current or the negative penetration current. The direct relationship of durations means cleaning is increased as penetration is increased. This relationship of durations may be scaled. Thus, TIG welding systems or welders feature a method to adjust the waveform balance between penetration and cleaning based upon the set current, either average or negative. This balance concept does not address a basic concern in TIG welding wherein the current of the positive half cycle merely increases as the set current increases. Thus, at high currents, there is a tendency for the tungsten electrode to “spit” molten metal from the tip of the electrode into the melted puddle on the workpiece. This event is not acceptable in high quality TIG welding.